In a prior art antilock brake system, any excessive slip of wheels is detected by comparing the slip speed of each wheel, or a control variable calculated from its differentiated value, with a predetermined threshold value, the wheel speeds are controlled by increasing or reducing the brake pressure based on the result of the comparison, and the brake pressure on any wheel which has recovered from the locking state is increased at a fixed rate irrespective of the behavior of the wheel.
In order to detect a locking tendency of a wheel, it is an effective measure to carry out antilock control by comparing the slip speed, of each wheel or a control variable based on its differentiated value, with a predetermined threshold value as with the aforementioned prior art antilock brake system. But it is difficult to detect an excessively low slip speed resulting from an insufficient braking force when the wheels have recovered from the locking state and the wheel speeds have become substantially equal to the vehicle speed. Namely, during antilock control while the vehicle is moving, the vehicle deceleration is liable to drop below a predetermined value. This not only causes vibrations and pitching of the vehicle body but also leads to an extension of the braking distance.